By the autumn of 1917 the North Sea had become a slow, grinding war of mines. The British used them defensively to protect their convoys and offensively to choke off the German U-boats and surface raiders. The arithmetic worked in their favour. During 1917 alone, six U-boats were sunk by British mines, and over two years the German minesweeping effort had cost the Kaiserliche Marine roughly twenty-eight destroyers and seventy minesweepers and auxiliary vessels. To get warships in and out of the High Seas Fleet's bases, the Germans had been forced to sweep ever-widening lanes through the British minefields between Horns Reef and Terschelling. These sweeps - politely called Test Trips - were elaborate operations with minesweepers, torpedo boats, U-boats, barrier breakers, light cruisers, and sometimes a covering force of capital ships. They were also, from a British perspective, exactly the kind of large, scheduled, predictable event that an ambush was designed to catch.

On 20 October 1917 the British codebreakers of Room 40 - the Naval Intelligence Division's cryptographic operation - decrypted orders sent to the German submarine UB-61. Agent reports from Copenhagen filled in the rest. The Admiralty knew a major German Test Trip was coming. By mid-November they had enough to set up an interception, provided the British ships at Rosyth, in Scotland, could sail in time. The German operation was set for 17 November, and orders for a British ambush went out to Admiral Sir David Beatty, commander-in-chief of the Grand Fleet. The problem was internal. Room 40's charts showed not only the predicted German positions but the positions of the British minefields the Germans were trying to clear. That information was being added to Room 40's situation maps. It was not being given to Napier, the officer who would actually do the fighting. The Admiralty's terror of compromising the codebreaking source - of letting the Germans realise their wireless ciphers had been broken - meant the man on the bridge fought half-blind.

Just west of Sylt, on the morning of 17 November, British cruisers under Napier sighted the German minesweeping group and opened fire. Rear Admiral Ludwig von Reuter - the same Reuter who would later, as a prisoner at Scapa Flow, give the order to scuttle the German fleet - immediately advanced with four light cruisers and eight destroyers to cover the minesweepers' withdrawal. All the German minesweepers except a single trawler escaped. The German light forces poured smoke into the gap behind them and ran south-east at their best speed. The 1st Cruiser Squadron, the 1st Light Cruiser Squadron, and the 6th Light Cruiser Squadron chased them through the smoke, hampered on both sides by minefields they could not safely cross. Then the Kaiser-class battleships Kaiser and Kaiserin came up in support of Reuter's force, and the British found themselves under fire from ten-gun capital ships. One British cruiser took a shell through her upper conning tower that killed everyone there and mortally wounded her captain, Herbert Edwards. The gunnery officer, Lieutenant H. C. C. Clarke, took command from below with the electrical communications cut and the rate of fire reduced. Repulse, detached from the 1st Battle Cruiser Squadron, came up at speed and hit one of the German light cruisers with a shell that started a serious fire.

When the British reached the edge of yet more minefields, they broke off the chase. Admiral Reinhard Scheer later listed the German losses as twenty-one killed, ten seriously wounded, and thirty lightly wounded. The Admiralty communiqué put British losses at one officer and twenty-one men killed, four officers and forty-three wounded. Aboard Commodore Walter Cowan's flagship Caledon, Able Seaman John Carless of Walsall continued to load and fire his gun after shrapnel had torn open his abdomen. He died at his station and was awarded a posthumous Victoria Cross. The historian Patrick Beesly, writing in 1984, judged that the British operation had been daring and that Napier had been unjustly blamed for failing to press the attack home. He had been ill-informed, Beesly wrote, because the Admiralty would not share what Room 40 knew. "At the least," Beesly concluded, "Room 40 had prevented the British operation degenerating from fiasco to disaster."

The Second Battle of Heligoland Bight settled nothing tactically. The Germans got most of their minesweepers home. The British made it back to Rosyth with damaged cruisers and one Victoria Cross to bestow on a dead Walsall sailor. But the battle did clarify what the next year of war in the North Sea would look like: dense minefields choking the Bight, the Germans forced to scrape narrow corridors through them, the British waiting to strike at the moments when the corridors were known. Six months later, on 27 March 1918, HMS Ariel and her sister destroyers would catch three German armed trawlers in exactly such a corridor northwest of Heligoland. The patrols, the test trips, the minelaying, and the ambushes would continue until the war ended - and in some places, where unexploded mines from this war still lie on the seabed, they have not entirely ended yet.